std::unordered_set<Key,Hash,KeyEqual,Allocator>:: count

1) Returns the number of elements with key that compares equal to the specified argument key , which is either ​ 0 ​ or 1 since this container does not allow duplicates.

2) Returns the number of elements with key that compares equivalent to the specified argument x . This overload participates in overload resolution only if Hash :: is_transparent and KeyEqual :: is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key .

1) Number of elements with key key , that is either 1 or ​ 0 ​ .

2) Number of elements with key that compares equivalent to x .

#include <algorithm>
#include <iostream>
#include <unordered_set>
 
int main()
{
    std::unordered_set set{2, 7, 1, 8, 2, 8, 1, 8, 2, 8};
 
    std::cout << "The set is: ";
    for (int e : set)
        std::cout << e << ' ';
 
    const auto [min, max] = std::ranges::minmax(set);
 
    std::cout << "\nNumbers from " << min << " to " << max << " that are in the set: ";
    for (int i{min}; i <= max; ++i)
        if (set.count(i) == 1)
            std::cout << i << ' ';
    std::cout << '\n';
}

The set is: 8 1 7 2
Numbers from 1 to 8 that are in the set: 1 2 7 8